1. Field of the Invention
The present invention relates to an ultrasonic distance measure for estimating the distance to an object by measuring the propagation time of an ultrasonic wave. The present invention also relates to an ultrasonic transducer for use in the ultrasonic distance measure.
2. Description of the Related Art
An ultrasonic distance measure transmits an ultrasonic wave toward an object and receives the ultrasonic wave that has been reflected from the object, thereby measuring the propagation time of the ultrasonic wave and calculating the distance to the object based on the propagation time. An ultrasonic wave propagates through a gas or a liquid. That is why if the velocity at which the ultrasonic wave is propagating through the medium is known, then the distance to the object in the air or in the liquid can be calculated.
According to this principle of measurement, it is important for an ultrasonic distance measure to accurately define when a wave reflected from an object returned to the distance measure. In general, when the intensity of the reflected wave received reaches a predetermined sensing level, the ultrasonic distance measure acknowledges the sensing of the reflected wave and defines the time when the reflected wave arrived.
The ultrasonic wave attenuates while propagating through the medium, and therefore, the intensity of the reflected wave received changes according to the distance to the object. That is why according to a known technique, to sense a reflected wave accurately, a plurality of sensing levels are set for respective distances to measure such that a relatively high sensing level is adopted to measure a short distance and that a relatively low sensing level is adopted to measure a long distance. On the other hand, according to another known technique, the rise of a reflected wave received is accurately sensed by setting a plurality of sensing levels at a time. According to yet another technique, the waveform of an ultrasonic wave to transmit is given a special mark by inverting its phase, for example, and the arrival of its reflected wave is sensed by detecting that mark.
Also, in measuring with an ultrasonic distance measure, the accuracy of measurement is affected by fluctuation of the medium or surrounding noise. To avoid such an influence, Japanese Patent Application Laid-Open Publication No. 7-104063 discloses an ultrasonic distance measure for measuring the propagation time of an ultrasonic wave by subjecting an ultrasonic wave to transmit to frequency modulation and a signal representing a reflected wave received to correlation processing, respectively. As shown in FIG. 1, in such an ultrasonic distance measure, a carrier signal generating section 8 generates a carrier signal 8S as a burst wave and a modulating signal generating section 44 generates a modulating signal 44S for changing the frequency of the burst wave from f0 to f1 within a period T. A frequency modulating section 7 modulates the frequency of the carrier signal 8S with the modulating signal 44S and then outputs the resultant signal to a transmitting amplifier 3. In response to a signal from the modulating signal generating section 44, the transmitting amplifier 3 outputs a drive signal 3S to a transmitting ultrasonic transducer 38.
The ultrasonic wave, transmitted by the transmitting ultrasonic transducer 38 in response to the drive signal 3S, is reflected from an object 17. And the reflected wave reaches a receiving ultrasonic transducer 39. A received signal, representing the reflected wave received at the receiving ultrasonic transducer 39, is amplified by a receiving amplifier 4 to be a received signal 4S. Then, a frequency demodulating section 40 demodulates the received signal 4S.
Next, a cross-correlating section 41 sequentially causes a delay in the modulating signal 44S at regular time intervals since the time when that signal was generated, and obtains a cross-correlation function 41S representing a correlation between the delayed signal and the demodulated received wave. A degree of correlation detecting section 43 detects a time delay with the highest degree of correlation as a propagation time in the cross-correlation function 41S.
The apparatus disclosed in Patent Document No. 1 uses a frequency-modulated ultrasonic wave, and its ultrasonic transducers need to operate in a relatively wide frequency range. However, an ultrasonic transducer generally used produces an ultrasonic wave by utilizing the resonance phenomenon of a piezoelectric ceramic. Thus, according to its operating principle, such an ultrasonic transducer can produce an ultrasonic wave only within a narrow range in the vicinity of its resonant frequency. Accordingly, if the frequency of an ultrasonic wave should be modulated in a range that is broad enough to avoid the effects of noise sufficiently, then it would be difficult to drive the ultrasonic transducers all through that broad range.
Some ultrasonic transducers can operate in a wide frequency range. However, such ultrasonic transducers need to be driven with a high output and are hard to drive with a battery, for example. Also, if the frequency range broadens, then the ultrasonic transducer on the receiving end is likely to collect noise, which is a problem.